The Nonadiabaticity Question for Europium(III/II): Outer-Sphere Reactivities of Europium(III/II) Cryptates

The one-electron reduction kinetics of the europium cryptates Eu(2.2.1)³⁺ and Eu(2.2.2)³⁺ by the aquo ions [formula-omitted], and [formula-omitted] and the oxidation kinetics of Eu(2.2.1)²⁺ by [formula-omitted] have been studied by using a polarographic technique in order to examine the effects of encapsulating europium within cryptate cavities upon the reactivity of the Eu(III/II) couple. At 25 °C and an ionic strength m = 0.1, the second-order rate constants (M^-1 s^-1) for acid-independent pathways are as follows: [formula-omitted]. By comparison of these kinetic data with those for similar reactions involving the [formula-omitted] couple, the rate constant for Eu(III/II) self-exchange, is estimated to increase by factors of ca. 1 X 10⁷ and 2 X 10⁴ upon encapsulation of europium in (2.2.1) and (2.2.2) cryptate cavities, respectively. Estimates of ka equal to ca. 10, 4 X 10^-2, and 5 X 10^-6 M^-1 s^-1 (μ = 0.1) for Eu(2.2.1)^3+/2+, Eu(2.2.2)^3+/2+, and [formula-omitted], respectively, are obtained from the Marcus cross relation. The increases in ka resulting from cryptate encapsulation suggest that nonadiabaticity is not primarily responsible for the extremely low reactivity of [formula-omitted]. The values of kex are shown to be roughly consistent with the Franck-Condon barriers estimated from structural data.